This proposal is based on two novel findings that identify both Sox9 and Hox5 as critical players in lung development. In addition to identifying novel roles for these transcription factors in early lung patterning and branching morphogenesis, our preliminary evidence suggests that Hox5 regulates Wnt signaling in the mesenchyme to control mesenchyme-epithelial crosstalk upstream of Sox9 expression in both the mesenchyme and epithelium. By identifying novel molecular mechanisms by which Hox5 and Sox9 regulate lung development, the proposed work will add significant new data to our existing understanding of lung organogenesis. Furthermore, the proposed experiments are designed to synthesize a comprehensive view of how Sox9 and Hox5 integrate with the Wnt/?-catenin signaling pathway to regulate early events in lung development, including patterning, branching morphogenesis, proliferation and differentiation. In order to fully elucidate this novel Hox-Wnt-Sox signaling axi, we will define Hox5- and Sox9-mediated molecular regulation of lung development and determine the mechanisms by which Hox5 modulates Wnt/?-catenin signaling to control mesenchymal-epithelial crosstalk and Sox9 expression.